Currently there is considerable interest and debate as to whether inverse agonists represent merely a pharmacological curiosity or whether they have physiologically relevant actions and therapeutic potential. The debate arose because inverse agonism has been demonstrated primarily in artificial systems where receptors are overexpressed or mutated (constitutively active mutants). In these non-natural systems, inverse agonists typically reduce "basal" effector activity as a result of reducing ligand-independent (constitutive) receptor activity. However, evidence is beginning to emerge which suggests that inverse agonists have novel actions that extend beyond simply reducing basal effector activity and that these novel effects occur in systems where non-mutated receptors are expressed at natural densities. We have found that even though 5-HT2C inverse agonists do not reduce basal effector activity in CHO cells expressing low levels of the human 5-HT2C receptor, they enhance the responsiveness of both the 5-HT2C receptor system and the endogeously expressed purinergic receptor system. These data suggest that cellular actions of inverse agonists may be mediated through activation of receptor systems that are not direct targets for these drugs. Since many therapeutic and investigational drugs that were previously thought to be antagonists have recently been demonstrated (or are suspected) to be inverse agonists, these novel actions may be important for their therapeutic efficacy. To explore these novel actions of inverse agonists in physiologically relevant model systems, we propose to: 1) test the hypothesis that 5-HT2A and 5-HT2C receptors are constitutively active in brain and when expressed at natural densities in cell culture models. Further, we will test the hypothesis that these receptor systems are partially desensitized due to subsensitivity of components of the effector system; and 2) test the hypothesis that ligand-independent 5-HT2A and 5-HT2C receptor activity, in physiologically relevant model systems, leads to constitutive desensitization of other non- target receptor systems that share the same post-receptor signaling components. We will measure the capacity of 5-HT2A/2C inverse agonists to alter responses to activation of receptors which are not direct targets for 5-HT2A/2C inverse agonists (e.g. purinergic, muscarinic, and adrenergic receptors), but which are known to activate the same cellular effector pathways. This study will provide important information about the cellular effects of inverse agonists which should help us to better understand the actions of a variety of therapeutic and investigational drugs.